Ischemic injury causes an immediate and marked fall in brain pH, the consequences of which are multiple and complex and incompletely understood. The recent discovery of proton-gated cation channels (acid-sensing ion channels, ASIC) in the brain offers a novel opportunity to explore hitherto unknown aspects of biology relevant to understanding the response of the brain to ischemia. The ASIC system does respond during ischemia. Our preliminary data show up-regulation of the brain-specific ASIC2a subunit following ischemia and co-localization of another ASIC subunit to brain mitochondria, a major site of perturbation during ischemic necrosis and apoptosis. In patch-clamping experiments, cultured neocortical cells showed an acid-evoked current matching that of ASIC1a, whose properties changed after in vitro ischemia, and neuronal (PC12) cells displayed an acid-evoked current that was blocked with an ASIC subunit-specific antagonist. We therefore propose to 1) characterize the expression of all ASIC subunits after focal and global ischemia; 2) identify and compare ASIC subunit-protein interactions in normal and ischemic brain; 3) describe the sub-cellular location of the ASIC subunits; and 4) assess electrophysiologically the effect of ischemia on ASIC function and the effect of ASIC activity on the outcome of ischemic injury. These novel studies are likely to offer new understanding of the biology of brain ischemia and offer new targets for therapeutic intervention.